It is well known to use screws to fasten implantable parts such as bases used to fasten joint prosthesis components or cortical or similar plates serving to perform an arthrodesis or osteosynthesis of one or more bones.
A very traditional base or plate comprises smooth cylindrical holes in which the fastening screws are designed to be engaged. Each hole may be adjusted to the dimension of the screw designed to be engaged in that hole, thereby defining a single implantation direction of the screw, or may have a diameter larger than that of the screw, to allow implantation of the screw in a plurality of implantation directions.
In the first case, the base of the plate has the drawback of imposing an implantation direction on the screw and therefore prohibiting implantation directions other than that which is imposed. However, depending on the anatomy of the bone or condition of the patient, it may be preferable to implant the screw in a different direction from that of the hole. In the second case, the base or plate does not rule out defective positioning of the screw in relation to the bone(s) in which it is implanted, in particular the risk of the screw emerging at a joint surface of one or more of said bones.
It is also known to arrange a tapping in the hole and a thread on the screw head, the thread of the screw engaging with the tapping of the hole. Such assemblies also have the drawback of imposing an implantation direction on the screw determined by said engagement.
Furthermore, such assemblies have a noteworthy risk of unscrewing of one or more screws over time under the effect of the repeated forces undergone by the bone(s). To resolve this risk of unscrewing, various axial retention systems have been designed for the screws, which, in general, have the drawbacks of being relatively expensive to manufacture and not always being very effective.
Patent application publications nos. US 2010/312286, US 2008/208259 and US 2008/140130 illustrate various techniques of the prior art.
The present invention aims to resolve the aforementioned drawbacks.
The main aim of the invention is to provide an assembly as previously mentioned, in which at least one screw can be implanted in an implantation direction chosen from among several possible implantation directions, and in which the risk of unscrewing of that screw over time is reduced.
Another aim of the invention is to provide an assembly in which the risk of defective positioning of a screw is ruled out, or preserving the possibility of choosing from among several possible implantation directions.